1. Technical Field
The present invention relates to an imaging apparatus, an imaging method, and a focus control apparatus.
2. Related Art
Many imaging apparatus of related art, such as a monitoring camera and a video camcorder, have a built-in autofocus function that allows automatic focus adjustment. An example of a focusing method in an autofocus function of this type is a contrast-based focus adjustment method using a fact that the amplitude of a contrast signal provided from captured video images is maximized when the images are brought into focus.
In an imaging apparatus, when a focusing lens is moved along the optical axis thereof, captured video images are defocused or brought into focus and the amplitude of the contrast signal changes accordingly. In a most fundamental contrast-based method, the focusing lens is temporarily moved along the optical axis thereof, and a direction in which images are brought into focus is detected based on the magnitude of the amplitude of the contrast signal before and after the movement, followed by movement of the focusing lens in the detected direction.
An example of a background technology in the present technical field is JP-A-2006-023339. JP-A-2006-023339 describes “A digital camera 1A includes an evaluation section that evaluates based on image data whether or not a scene under imaging is a night scene. The evaluation section determines the scene under imaging is a night scene when a high-luminance portion is present in a first portion (such as AF area AR) that is a central portion of a framing region FR and a low-luminance portion is present in a second portion (such as hatched area MR) in the framing region” (see Abstract).
When an image of an intense light source, such as a headlight of an automobile and a street lamp, is captured, phenomenon in which the amplitude of the contrast signal is undesirably maximized in a false focus position where an in-focus state is not achieved occurs, as shown in FIG. 12.
FIG. 12 shows values of the contrast signal versus the focusing lens position in a case where a point light source is present in a subject. As, shown in FIG. 12, when a point light source is present in a subject, a false peak appears in some cases in a position shifted from a correct in-focus-position toward the Near side and/or in a position shifted from the correct in-focus position toward the Far side. Therefore, when a point light source is present, an effect of the false peak prevents in some cases the focusing lens from reaching the correct in-focus position where an image of the subject is brought into focus.
In autofocus control performed when a point light source is present, an out-of-focus state undesirably occurs during zoom-in or zoom-out operation because the effect of the false peak prevents the focusing lens from reaching the correct focus position. At nighttime, in particular, when a state in which an image of a wide-range scene is captured at a low magnification is changed to a state in which the registration plate of a car having headlights is zoomed in or to a state in which a subject close to a street lamp is zoomed in, it is conceivable that an out-of-focus state occurs.
JP-A-2006-023339 describes that the focusing lens is moved in advance to a position corresponding to infinity before an image of a night scene is captured. JP-A-2006-023339, however, does not disclose how to move the focusing lens in the following possible zooming operation, and an out-of-focus state may occur during the zooming operation when a point light source is present.